Distillation is a widely known process for effecting the separation of a fluid mixture. In the process, at least one component of the mixture is removed from the distillation column as an overhead vapor fraction. The less volatile component is removed from the column as a bottoms liquid fraction. The efficiency of a distillation column for separating a mixture into its components is directly related to the vapor velocity of the gas in contact with the liquid. If the vapor velocity is too high at some points in the column, there is a tendency to entrain liquid droplets and carry the liquid component upwardly. If the vapor velocity is low or non-existent, then there is insufficient contact to effect separation. Thus, it is desirable to maintain a uniform vapor velocity at all points on a tray. Not only is it desirable to achieve a uniform vapor velocity over the tray it is also desirable to maintain a uniform liquid/vapor ration (L/V) at all points on a tray. Variations in L/V, commonly known as maldistribution can be is caused by variations in vapor flow rate, liquid flow rate, or both. Vapor flow variations are most commonly caused by variations in the liquid level above the perforations, i.e., liquid head on the sieve tray. Variations in liquid level are caused by non-uniform liquid flow path lengths, liquid flow rates, unlevelness of the tray, etc. Variations in liquid flow rate are most commonly caused by non-uniform flow path lengths, and by non-uniformities in the distribution of liquid as it enters the distillation tray.
Another factor which influences the efficiency of the separation is related to the concentration gradient of the components in both the liquid and the vapor. The cross-flow tray, which is a standard in the industry, suffers from the problem that the concentration gradient shifts from one side of the column to the other as the direction of liquid flow changes on successive trays.
A technique developed to maintain a more uniform concentration gradient in a distillation column has been utilized and this technique includes a helical liquid flow path. In this technique the tray has been divided into a plurality of wedge shaped sections and liquid caused to flow across the wedge shaped sections in a generally arcuate flow path (helical within the column). The liquid is then removed and introduced to the tray below with the flow pattern being approximately 40 to 170.degree. offset from the inlet to the tray above. The angle of offset varies approximately inversely with the number of segment zones. Although this type of tray maintains direction of liquid flow throughout the column, the trays suffer from maldistribution due to the common factors of variable flow path lengths, liquid recirculation, liquid gradient and so forth.
Representative patents which illustrates various trays for effecting liquid-gas contact in systems and designed to alter the flowpath of vapor or liquid on the tray as well as reduce the possibility of maldistribution caused by variable liquid levels above the perforations are as follows:
U.S. Pat. No. 3,759,498 discloses a slotted sieve tray having variable slot density. The trays incorporate a plurality of band members which are transverse to the liquid path and are designed to minimize the liquid gradient over the tray. The slot density within the band areas in the tray is varied to provide for a constant L/V over the entire tray.
U.S. Pat. Nos. 3,887,665; 4,105,723 and 4,132,761 discloses vapor liquid contacting devices having a damping means adjacent the surface of the tray to limit fluid oscillation and reduce vapor velocity through tray openings. One technique for damping fluid oscillation has been to include a layer of mesh comprising an interconnected cellular material. Typically knitted wire mesh, woven cloth and filamentary or fibrous felt for matting were used to accomplish this result. Parallel vertical sheets are located on a tray directly above the active area of the plate and used in conjunction with the knitted wire matrix to limit fluid oscillations.
U.S. Pat. No. 4,101,610 discloses a slotted sieve tray of the double pass type. A froth initiator is located at the inlet so as to reduce the kinetic energy of the liquid and thereby its hydrostatic head to allow full use of the contacting surface. Band portions adjacent to the liquid inlet and extending downstream for a distance of at least 20% of the transverse length of the liquid inlet and having angular orientation of slot openings away from the tray diametrical streamline are used.
British Pat. No. 765,623 discloses a tray for contacting a liquid with a gas having a corrugated surface and a liquid pathway generally transverse to the corrugations. A plurality of perforations are distributed in the corrugations with the vertical distance to the level of the liquid being non-uniform. In other words, some of the perforations are located near the upper portion of the corrugations while some are located in the valleys.